1. Field of the Invention
The present invention relates to a voltage detection circuit used for output voltage detection portions and so on for power supply units (such as switching power supplies), a power supply unit using the voltage detection circuit, and a semiconductor device used for the voltage detection circuit.
2. Background Art
FIG. 14 is a circuit diagram for showing a voltage detection portion used for a conventional power supply unit. FIG. 15 is a waveform graph for showing operation at each terminal in the conventional voltage detection portion shown in FIG. 14. FIG. 16 is a circuit diagram for showing an exemplary power supply unit using the voltage detection portion shown in FIG. 14.
According to the conventional voltage detection circuit, a voltage at an output voltage terminal VOUT is divided based on a resistance ratio of resistors 34 and 35 and is detected by a shunt regulator 32. A capacitor 13 is connected to the output voltage terminal VOUT and is externally supplied with a charge, which increases the voltage at the output voltage terminal VOUT. The detected voltage level VO at the output voltage terminal VOUT is expressed by the following equation:VO=Vth·(R34+R35)/R34wherein Vth denotes the threshold voltage of the shunt regulator 32, R34 denotes the resistance of the resistor 34, and R35 denotes the resistance of the resistor 35.
In order to stably control the detected voltage level VO, the following steps are necessary:
(1) the resistor 36 and the capacitor 37 are connected in series to each other as shown in FIG. 14; and
(2) an detected current value to the shunt regulator 32 is increased by additionally connecting a resistor 33 as shown in FIG. 14 in order to stabilize the detected voltage level.
FIG. 15 is a graph for showing the waveforms of current 132 which flows through the output voltage terminal VOUT, VO, and the shunt regulator 32 in the voltage detection circuit shown in FIG. 14. The ripple voltage at the output voltage terminal VOUT is determined by the time constant of the resistor 36 and the capacitor 37 (since it is necessary to give hysteresis characteristics).
FIG. 16 is a circuit diagram for showing an exemplary power supply unit wherein the conventional voltage detection circuit shown in FIG. 14 is used as a secondary voltage detection circuit. In FIG. 16, reference numeral 18 denotes a rectifier circuit, 19 denotes a transformer, 19a denotes the primary winding of the transformer, 19b denotes the secondary winding of the transformer, 20 denotes an input voltage source, 21 denotes a filter circuit, 22 denotes a rectifier circuit, 23 denotes an smoothing capacitor on the input side, 24 denotes a snubber circuit, 25 denotes a control circuit, 26 denotes a switching element, 27 denotes an section incorporated on a single semiconductor substrate, and 28 denotes a capacitor.
In this case, a optical coupler 14 is used as a detected signal transfer unit. When a voltage is detected by the conventional voltage detection circuit shown in FIG. 4, the shunt regulator 32 is brought into conduction, so that a current flows into the light emitting portion 14a of the optical coupler 14 to emit light (the output of the detected signal). The output of the detected signal taking the form of the light emission is detected by the light-receiving portion 14b of the optical coupler 14, and the on-off control of the switching element 26 effected by the primary control circuit 25 is stopped (or suspended). As a result, energy supply from the primary side to the secondary side is stopped (or suspended), which lowers the voltage at the output voltage terminal VOUT gradually. When the voltage at the output voltage terminal VOUT goes down to below the detection voltage level, the output of the detection signal from the optical coupler 14 and the shunt regulator 32 goes off, so that the on-off control of the switching element 26 effected by the control circuit 25 is resumed to supply energy from the primary side to the secondary side, which increases the voltage at the output voltage terminal VOUT. The ripple voltage at the output voltage terminal VOUT varies according to the loaded state of the input voltage terminal VOUT since the ripple voltage is dependent upon the time constant of the resistor 36 and the capacitor 37.
As to the voltage detection circuit, the following references are provided.    Reference 1: “Feature—All of the Latest Power Supply Circuit Designing Techniques”, Transistor Technology• Special Issue, CQ Press, Jul. 1, 1991, No. 28, p. 13.    Reference 2: “Feature—All of the Latest Switching Power Supply Techniques”, Transistor Technology• Special Issue, CQ Press, Jan. 1, 1997, No. 57, p. 86.
However, the conventional voltage detection circuit has the following problems.
(1) In order to increase voltage detection accuracy, it is necessary to increase the current value of the current 132 which flows into the shunt regulator 32. However, the current flowing during detection is generally on the order of several milliamperes, which interferes with the achievement of higher efficiency (that is, energy savings) being addressed on a worldwide basis at present. Also, in order not to decrease the voltage detection accuracy when the current value of the current I32 flowing into the shunt regulator 32 is insufficient, it is necessary to add the resistor 33, which also increase the number of components of the circuit in addition to the reason set forth below.
(2) Since the voltage control at the output voltage terminal VOUT is dependent on the time constant of the resistor 36 and the capacitor 37, the ripple voltage range at the output voltage terminal VOUT varies depending on the state of the load, which may cause increased ripple voltage.
(3) The components of the voltage detection circuit are large in number (at least eight components are required as shown in FIG. 14).
(4) Being dependent on the temperature characteristics of the threshold of the shunt regulator 32, the temperature characteristics of the voltage at the output voltage terminal VOUT are poor.